This invention relates to improvements in detection systems of the beam variety, and more particularly, to apparatus for solving the so-called "near-field" problem which characterize such systems.
Single-terminal, beam-type, detection systems, such as intruder detection systems, typically comprise a transceiver unit which transmits a beam of electromagnetic radiation (e.g. visible, infrared or microwave radiation) through a space in which a condition to be sensed is anticipated, and detects the intensity of the beam upon being reflected by a remotely positioned retroreflector. Movement of an intruder through the beam usually results in a sudden reduction or drop-out in the intensity of the received beam. This condition is sensed by conventional threshold-sensing circuitry, and an indication, such as an alarm, is given.
In installing detection systems of the type described above, it is common to set the threshold of the alarm-actuating circuitry at a relatively high level. Typically, the threshold is set at such a level that an alarm will not be sounded unless the intensity of the reflected beam drops to a relatively small percentage of its original intensity, such as, for example, 25% of its original intensity. Such a high margin of error is often necessary to prevent the effects of air turbulence and shimmer produced by thermal gradients between the transceiver unit and the remotely spaced retroreflector from activating the alarm.
Heretofore, intrusion detection systems of the type described above have, under certain circumstances, been unable to detect the movement of an intruder through the beam. For instance, if the intruder crosses the transmitted beam at a location in close proximity to the transceiver, he may reflect sufficient energy back to the transceiver to prevent the alarm-activating threshold from being reached. Whenever the intruder's presence in the beam fails to cause the reflected radiation level at the transceiver to drop below the threshold required for alarm actuation, the system will not detect the intruder's presence. This problem, of course, does not arise when the intruder is substantially removed from the transceiver since, regardless of his reflectivity, he would be incapable of reflecting sufficient radiation back to the receiver as to prevent alarm activation. The severity of this "near-field" problem increases as the intruder's reflectivity increases, as the aforementioned margin of error of the detection system increases, and as the point at which the intruder crosses the beam approaches the transceiver unit.
Another type of near-field problem which adversely effects the performance of intrusion detection systems of the above type is one which results from the use of a poorly collimated beam of radiation. Because safety regulations and other factors often prevent the use of the well-collimated beams of radiation produced by lasers and masers, conventional optical elements and wave guides must be used to control the directionality of the transmitted beam. Unfortunately, such devices allow the transmitted beam to gradually increase in diameter and thereby allow the beam to impinge upon objects between the transceiver and the retroreflector. Thus, it may be appreciated that in installing such systems, care must be taken in aiming the beam of radiation to avoid having radiation impinge upon objects of relatively high reflectivity located in relatively close proximity to the transceiver unit. If the beam strikes a reflective object in close proximity to the transceiver, such object may reflect more radiation back to the receiver than does the remotely spaced reflector, and a total drop-out in return signal from the reflector may not cause the alarm threshold to be reached.
To solve the above-identified near-field problems, it has been common to minimize the field of view of the receiver optics and to deliberately misalign the transmitter and receiver optics so that the reflector is just inside the field of view of the receiver. By this arrangement, radiation reflected by near-field objects will strike the receiver at such an oblique angle as to be outside its field of view. This technique works well in systems in which there is a rigid connection between the transmitter and receiver optics whereby the respective fields of view can be set at the factory. However, new competitive designs often require that the transmitter and receiver optics be independently pointable. This design results in greater flexibility in installations, and less expensive design and fabrication. In installing such systems, it is common for the installer to align the components for maximum signal return. As may be appreciated, such an alignment will not necessarily minimize near-field problems.
In single-terminal photoelectric detection systems having independent transmitter and receiver optics, it is possible to reduce the effects of near-field objects by physically separating such optics. The drawbacks of this approach are twofold. First, the appearance of the system, as well as the packaging thereof, becomes awkward. Secondly, widely spaced optics will not allow operation from a reflector at short range. Over a short range, the spreading of the beam from conventional reflectors may be so small that the return beam may not irradiate the receiver.